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The Webfooted Astronomer - August 2001

 

Minutes: Thar's Fossils in Them Thar Rilles, Part II

By Greg Donohue

BASED on Dr. Guillermo Gonzales' May presentation to the SAS, the June newsletter minutes explored the possibility of signs of very early terrestrial life being preserved in fragments of Earth meteorites on the Moon. If we suppose that such fragments do exist, where would they most likely be, and how would we go about finding them? And are there any other important consequences of this theory?

[For a report on the July SAS meeting, see the President's Pen on page 3—Editor]

In the first installment, I noted that during the lunar cataclysm 3.9 to 3.8 billion years ago, material ejected from the Earth due to asteroid impacts could reach the Moon by a direct trajectory, or by indirect orbital transfer. During this early time, the direct method was about 7 times more effective in transferring material to the Moon than was the indirect orbital transfer mechanism.

Calculating the distribution of the velocities with which direct transfer material impacted the Moon reveals that the far side receives the lowest velocity impacts. This material would be subjected to the least pressures, and thus be better preserved than the higher-velocity impact material on the Moon's near side. The far side is therefore a prime place to look for the best-preserved specimens. Conversely, the near side sweeps up more material, so Earth fragments should be more abundant there. Consequently, where you look depends on whether you are looking for the most stuff, or the most well preserved stuff. Dr. Gonzales suggests that somewhere in between the near and far sides might be a good compromise for finding relatively abundant, relatively well preserved samples.

Older structures on the Moon have had the most time to collect Terran meteorites, and have also experienced the most thorough mixing of the Lunar and Terran materials due to micrometeorite impacts. The maximum expected amount of Terran material would be about 1 part per thousand at the bottom of a 3.9 billion-year-old surface.

Given this information, where on the Moon would you tend to look for Terran meteorites? Dr. Gonzales prefers a large, smooth mare, in an area undisturbed by any recent large impacts. In such an area, you would simply dig down into the deepest part of the regolith, which is where you would expect to find the most Terran meteorites. So while the title of this article is hopefully a catchy one, lunar rilles are in fact probably not the best places to look for the fossilized remains of early microbial Earth life.

An alternate strategy for finding Terran meteorite fragments is to look on the surface of the Moon, close to a crater 10 to 20 meters across. Collisions that cause this size crater excavate down about 10 meters into the regolith. Thus this deeper material is brought up and scattered around the surface. There's no "heavy lifting" involved when looking for Terran meteorite traces in this manner. A small meteorite has done the excavation work for us. But this additional impact will likely shock the surrounding material. This added trauma might damage or destroy delicate specimens, including those that might harbor signs of early, microscopic Earth life.

However, collecting surface samples could be accomplished with a small, inexpensive, remotely controlled rover. The question becomes one of how to distinguish Terran meteorite fragments from the rest of the lunar material.

This could be accomplished in a number of ways. Material that has passed through an atmosphere of some type would have its surface melted, so the presence of an ablation crust would be a key indicator. Carbonates, which form only in the presence of water, fluoresce under ultraviolet radiation, so looking for this type of fluorescence could help locate Terran fragments.

Finally, looking for telltale radioactivity signatures unique to certain Earth rocks would be yet another way to locate potential Terran meteorites. A much more effective, but also far more expensive way to find and study Terran meteorite samples would be the establishment of a manned lunar base.

Astronauts would then become lunar prospectors, and actually mine for the desired material. The ultimate prize would be to uncover samples with microscopic pieces of early Earth algae or bacteria. Signs of such early life are no longer present on Earth. Our planet's geologic processes have long since recycled all material from that epoch, destroying any organic evidence.

The possibility that early Earth life-forms might have traveled as stowaways aboard fragments of rock blasted from Earth by large asteroid or comet impacts has very important consequences for the development and survival of life on Earth. The same huge asteroids that blasted Terran material to the Moon during the lunar cataclysm epoch were also likely to have been powerful enough to utterly obliterate any early Earth life. These gargantuan impactors, much larger than the one suspected of wiping out the dinosaurs 65 million years ago, had the capability to completely vaporize all of the Earth's oceans, and raise the surface temperature to hundreds of degrees, effectively sterilizing the entire planet.

How long would it take for the Earth to recover from these types of cataclysmic impact events? Some models predict a couple thousand years. Simulations by Dr. Gonzales' group reveal that material would continue to rain down on the Earth for several thousand years. Would it be possible for microbes inside this orbiting Terran ejecta to remain viable for this length of time? Again, calculations tend to show that they could. Dr. Gonzales believes that the Earth could indeed have been re-seeded with life after a sterilizing impact.

Mining the Moon for signs of early Earth life is a tantalizing prospect. And who knows? Perhaps in the not-too-distant future, lunar prospectors will work on the Moon. And of course their long lonely hours will undoubtedly give rise to new mining songs, possibly based on old Earth mining tunes:
"In a cavern, in a mare,
Excavating for a life-sign,
Dwelt a miner, forty-niner,
And his spacecraft Clementine.
Oh my darlin', Oh my darlin'.…"
Then again, maybe not....

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